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Study reveals human drive for fairness

24 August 2012

People will reject an offer of water,
even when they are severely thirsty, if they think the offer is unfair,
according to a new study by researchers at the Wellcome Trust Centre for Neuroimaging at
UCL. The findings have
important implications for understanding how we make decisions that need to
balance fairness and self-interest.

In studies that require people to share out money, it has
been known for some time that the person receiving an offer will tend to reject
it if they think it unfair, preferring to let both parties walk away with
nothing rather than accept a low offer in the knowledge that the other person
is taking home more.

In contrast, our closest relatives, chimpanzees - when
bargaining for food - will almost always accept an offer regardless of any
subjective idea of 'fairness'.

This study was designed to find out whether humans would similarly accept unfair offers if they
were bargaining for a basic physiological need, such as food, water or sex.

The team recruited 21 healthy participants and made 11 of
them thirsty by drip-feeding them a salty solution, while the others received
an isotonic solution that had a much smaller effect on their level of thirst.
The participants reported their own perceptions of how thirsty they were using
a simple rating scale, and to obtain an objective measure of each individual's
need for water, the team measured the salt concentration in their blood.

These findings show that humans, unlike even our closest relatives, chimpanzees, reject an unfair offer of a primary reward like food or water - and will do that even when severely thirsty.

Dr Nick Wright, Wellcome Trust Centre for Neuroimaging at UCL

The participants then individually took part in an ultimatum
game. They were told that two of them had been randomly selected to play a game
to decide the split of a 500 ml bottle of water that could be consumed
immediately. One of them would play the part of 'Proposer' and decide how the
bottle should be split. The other would be a 'Responder' who could either
accept the split and so drink the offered water, or reject the split so that
both parties would get nothing. The participants knew that they would have to
wait a full hour after the end of the game before they would have another
chance to drink anything.

In reality, all of the participants played the part of the
Responder. They were presented with two glasses of water with a highly unequal
offer that they were told was from the Proposer: the glass offered to them
contained 62.5 ml, an eighth of the original bottle of water, and the other
contained the remaining seven-eighths that the Proposer wanted to keep for
themselves. They had 15 seconds to decide whether to accept or reject the
offer.

The team found that, unlike chimpanzees, the human
participants tended to reject the highly unequal offer, even if they were
severely thirsty. The participants' choices were not influenced by how
objectively thirsty they were, as measured from the blood sample. However, they
were more likely to accept the offer if they subjectively felt thirsty.

Dr Nick Wright, who led the study, says: "Whether
or not fairness is a uniquely human motivation has been a source of
controversy. These findings show that humans, unlike even our closest
relatives, chimpanzees, reject an unfair offer of a primary reward like food or
water - and will do that even when severely thirsty. However, we also show this
fairness motivation is traded off against self-interest, and that this self-interest
is not determined by their objective need for water but instead by their
subjective perception of thirst. These findings are interesting for
understanding how subjective feelings of fairness and self-interested need
impact on everyday decisions, for example in the labour market."

The study is published online today in the open access
journal 'Scientific Reports'.